Apparatus for continuous production of polymers in carbon dioxide

ABSTRACT

A method for carrying out the continuous polymerization of a monomer in a carbon dioxide reaction medium comprises the steps of: (a) providing an apparatus including a continuous reaction vessel and a separator; (b) carrying out a polymerization reaction in the reaction vessel by combining a monomer and a carbon dioxide reaction medium therein (and preferably by also combining an initiator therein), wherein the reaction medium is a liquid or supercritical fluid, and wherein the reaction produces a solid polymer product in the reaction vessel; then (c) withdrawing a continuous effluent stream from the reaction vessel during the polymerization reaction, wherein the effluent stream is maintained as a liquid or supercritical fluid; then (d) passing the continuous effluent stream through the separator and separating the solid polymer therefrom while maintaining at least a portion of the effluent stream as a liquid or supercritical fluid; and then (e) returning at least a portion of the continuous effluent stream to the reaction vessel while maintaining the effluent stream as a liquid or supercritical fluid. The need for significant recompression of the continuous effluent stream prior to return to the reaction vessel is thereby minimized. Apparatus for carrying out such methods is also disclosed.

CLAIM FOR PRIORITY AND CROSS-REFERENCE TO OTHER APPLICATIONS

This application claims priority to and is a divisional of parentapplication No. 09/709,206, filed on Nov. 9, 2000 and issued as U.S.Pat. No. 6,914,105, which claims priority to Provisional ApplicationSer. No. 60/165,177, filed Nov. 12, 1999, the disclosures of which areincorporated herein by referenced in their entirety.

FIELD OF THE INVENTION

The present invention concerns methods and apparatus for the continuousproduction of polymers in a carbon dioxide reaction medium.

BACKGROUND OF THE INVENTION

Increased environmental concerns and regulations over the use ofvolatile organic compounds (VOCs) since the late 1980s (e.g. MontrealProtocol in 1987 and the Clean Air Act amendments in 1990) have causedconsiderable effort being put into finding environmentally benignsolvents for industrial use (McHugh, M. A. and V. J. Krukonis,Supercritical Fluid Extraction: Principles and Practice. Second ed, ed.H. Brenner. 1994, Boston: Butterworth-Heinemann). DeSimone et al. at theUniversity of North Carolina-Chapel Hill have shown that supercriticalcarbon dioxide (scCO₂) is a viable and promising alternative solvent(T_(c)=31.8° C., P_(c)=76 bar) to perform free-radical, cationic andstep-growth polymerizations using batch reactors (DeSimone, J. M., Z.Guan, and C.S. Elsbernd, Synthesis of Fluoropolymers in SupercriticalCarbon Dioxide. Science, 1992. 257: p. 945-947). This work has beensummarized in several recent reviews (Kendall, J. L., et al.,Polymerizations in Supercritical Carbon Dioxide. Chem.Rev., 1999. 99(2):p. 543-563; Canelas, D. A. and J. M. DeSimone, Advs. Polym. Sci., 1997.133: p. 103-140; Shaffer, K. A. and J. M. DeSimone, ChainPolymerizations in Inert Near and Supercritical Fluids. Trends inPolymer Science, 1995. 3(5): p. 146-153). Indeed, CO₂ technology isintended to be commercially implemented by 2006 for the manufacture ofTeflon™ by DuPont (McCoy, M., DuPont, UNC R&D effort yields results, inChemical & Engineering News. 1999. p. 10). The reasons for the intenseindustrial interest are that CO₂ is cheap ($100-200/ton), of lowtoxicity, non-flammable, and environmentally and chemically benign. Incomparison to existing technologies for making polymers, CO₂ technologyhas several significant advantages as it will allow for the eliminationof: a) expensive polymer drying steps; (b) expensive wastewatertreatment and disposal steps where significant amounts of monomer,surfactants and emulsifiers are generated (Baker, R. T. and W. Tumas,Toward Greener Chemistry. Science, 1999. 284: p. 1477-1478); (c)disposal of “spent” organic solvents; (d) handling, storage and shippingof toxic organic solvent; and (e) chain transfer to solvent, i.e., areaction that may limit the achievable molecular weight of the polymer.

As industrial interest in using scCO₂ as a polymerization medium hasgrown, several disadvantages of batch reactors have been recognized,including: (1) large reactors which are costly at the high pressures ofscCO₂; and (2) difficulty in recycling the CO₂ and the unreactedmonomer. Accordingly, there is a need for new ways to carry out thecontinuous polymerization of monomers in carbon dioxide, particularlyliquid and supercritical carbon dioxide.

SUMMARY OF THE INVENTION

The present invention provides a continuous process for theprecipitation polymerization of various monomers in scCO₂. In theprocess, recycling of the CO₂ without significant recompression isachieved. A continuous process requires smaller and hence cheaperequipment for large volume, commodity-based polymers. Moreover,continuous removal of polymer from the system, and recycling of monomerand supercritical fluid can be facilitated in a continuous system. Asmost organic monomers are soluble in liquid and scCO₂ (Journal ofOrganic Chemistry, 1984, vol. 49 pgs. 5097-5101) whereas most polymersare insoluble in CO₂, this process can be utilized for polymerization ofa great variety of monomers without the necessity of utilizing anyexpensive surfactant. A continuous system in scCO₂ can also be exploitedto incorporate in situ steps to purify the resultant polymer bysupercritical fluid extraction (SFE) (McHugh, M. A., Krukonis V. J.Supercritical Fluids Extraction: Principles and Practice.Butterworth-Heineman, Stoneham, 1993).

A first aspect of the present invention is, accordingly, a method forcarrying out the continuous polymerization of a monomer in a carbondioxide reaction medium. The method comprises the steps of:

-   -   (a) providing an apparatus including a continuous reaction        vessel and a separator;    -   (b) carrying out a polymerization reaction in the reaction        vessel by combining a monomer, an initiator, and a carbon        dioxide reaction medium therein, wherein the reaction medium        comprises liquid or supercritical carbon dioxide and wherein the        reaction produces a solid polymer product in the reaction        vessel; then    -   (c) withdrawing a continuous effluent stream from the reaction        vessel during the polymerization reaction, wherein the effluent        stream is maintained as a liquid or supercritical fluid; then    -   (d) passing the continuous effluent stream through the separator        and separating the solid polymer therefrom while maintaining at        least a portion of the effluent stream as a liquid or        supercritical fluid; and then    -   (e) returning at least a portion of the continuous effluent        stream to the reaction vessel while maintaining the at least a        portion of the effluent stream as a liquid or supercritical        fluid (and preferably at a pressure not more than about 50 or        100 psi less than the pressure in the reaction vessel), whereby        the need for significant recompression of the continuous        effluent stream prior to return to the reaction vessel is        minimized.

In a preferred embodiment, the combining of the monomer, the initiator,and the carbon dioxide reaction medium as recited by step (b) ispreferably carried out by continuously feeding the monomer, theinitiator, and the carbon dioxide reaction medium to the reactionvessel. In another preferred embodiment, a purge is positioneddownstream of the reaction vessel (described in detail herein) so as toremove an amount of the effluent stream as deemed appropriate by oneskilled in the art. In these preferred embodiments, the at least aportion of effluent that is returned to the reaction vessel is typicallya fraction less than one.

A second aspect of the present invention is an apparatus for thecontinuous polymerization of a monomer in carbon dioxide. The apparatuscomprises a continuous reaction vessel; at least one inlet lineconnected to the reaction vessel; an effluent line connected to thereaction vessel; a separator connected to the effluent line; a returnline connecting the separator to the reaction vessel so that liquid orsupercritical reaction medium is returned to the reaction vessel fromthe separators; and a control system or the like serving as a controlmeans for maintaining the reaction medium as a liquid or supercriticalfluid in the separator and the return line (and preferably at a pressurenot more than 50 or 100 psi different from the pressure in the reactionvessel during polymerization of monomer therein). In one embodiment,solid polymer is retained in the separator.

The present invention may also be carried out with any suitablepolymerization reactions, including but not limited to precipitation,microemulsion, emulsion, suspension, and dispersion polymerizationreactions. Any suitable initiator may be used, preferably one that issoluble in the liquid or supercritical reaction medium. Also, theinitiator that is not consumed in the reaction vessel is preferablyreturned to the reaction vessel in the reaction medium after the step ofpassing the effluent stream through the separator. The recycling of theinitiator is particularly desirable, since initiators are frequentlytoxic. Suitable separators include filters, cyclone separators, andseparators containing rotating devices therein, as described in greaterdetail herein. If desired a cooler may be positioned on the effluentline between the reaction vessel and the control valve. A recirculationpump may be positioned on the return line between the separator (or, ifa plurality of separators are employed, the separator positionedfurthest downstream from the reactor) and the reaction vessel, or at anyother suitable location. A condenser may be positioned on the returnline between the first and second separators and the reaction vessel.

The present invention is explained in greater detail in the drawingsherein and the specification set forth below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a small-scale continuous polymerizationapparatus employing filter separators, without recycle of the reactionmedium.

FIG. 2. GC Analysis for the Attainment of Steady State forPolymerization of VF2. The points are experimental data. Thepolymerization conditions were P=276 bar, T=75° C., ν_(CO2)=26 g/min,[VF2]_(INLET)=0.77 M, [DEPDC]_(INLET)=3 mM, and τ=21 minutes.

FIG. 3. Effect of agitation on the monomer conversion (X). The pointsare experimental data and the line is a linear least-squares regressionfit to the points. The polymerization conditions were P=276 bar, T=75°C., ν_(CO2)=26 g/min, [VF2]_(INLET)=0.77 M, [EPDC]_(INLET)=3 mM, andτ=21 minutes. ♦=Dispersimax™ Impeller, ▪=Upward pumping impeller.

FIG. 4. Plot of R_(p) versus [VF2]_(OUT) ^(1.0) to show first orderdependence of polymerization rate on monomer concentration. The pointsare experimental data and the line is a linear least-squares regressionfit to the points. The polymerization conditions were P=276 bar, T=75°C., ν_(CO2)=26 g/min, [EPDC]_(INLET)=3 mM, and τ=21 minutes.

FIG. 5. Effect of inlet initiator concentration, [I]_(IN), on themonomer conversion (X). The points are experimental data. Thepolymerization conditions were P=276 bar, T=75° C., ν_(CO2)=26 g/min,[VF2]_(INLET)=0.77 M, and τ=21 minutes.

FIG. 6. Plot of R_(p)/[VF2]_(OUT) versus [I]_(OUT) ^(0.5) to show squareroot dependence of polymerization rate on initiator concentration. Thepoints are experimental data and the line is a linear least-squaresregression fit to the points. The polymerization conditions were thesame as FIG. 5.

FIG. 7. Plot of ln k_(p)/k_(t) ^(0.5) versus 1/T to show fit withrespect to kinetic analysis. The line is a linear least-squaresregression fit to the points. The polymerization conditions wereρ_(CO2)=0.74 g/ml, ν_(CO2)=26 g/min, [VF2]_(INLET)=0.77 M,[DEPDC]_(INLET)=3.0 mM, and τ=21 minutes.

FIG. 8. Effect of polymerization temperature on the rate ofpolymerization (R_(p)). The points are experimental data. The line ismodel equation 12 for the R_(p). The experimental conditions areprovided in FIG. 7.

FIG. 9. Effect of mean reactor residence time, τ, on the rate ofpolymerization (R_(p)). The points are experimental data. The line ismodel equation 12 for the R_(p). The polymerization conditions wereP=276 bar, T=75° C., ν_(CO2)=26 g/min, [VF2]_(INLET)=0.77 M and[DEPDC]_(INLET)=3.0 mM.

FIG. 10. Parity plot showing the fit of all experimental data to thatpredicted from model equation 12.

FIG. 11. Effect of outlet monomer concentration, [VF2]_(OUT), on thenumber and weight average molecular weights, M_(N) and M_(W) determinedexperimentally by GPC. The points are experimental data. Thepolymerization conditions are the same as FIG. 8.

FIG. 12 is a schematic diagram of a large-scale continuouspolymerization apparatus employing cyclone separators, with recycle ofthe reaction medium.

FIG. 13 is a schematic diagram of a separator in the form of a pluralityof parallel filters that allows for collection of polymer and recycle ofreaction medium in accordance with a method of the invention.

FIG. 14 is a schematic diagram of a separator in the form of acontinuously stirred device that allows for collection of polymer andrecycle of reaction medium in accordance with a method of the invention.

FIG. 15 is a schematic diagram of a separator in the form of a cyclonein combination with a filter which serves to separate polymer from thereaction medium, allowing for the reaction medium to be recycled.

FIG. 16 is a graph comparing the dimensionless exit age distributionfunction E(θ) versus θ for an ideal CSTR and an experimental reactor.

FIG. 17 is a schematic diagram of a continuous loop reactor which may beemployed in accordance with the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention may be carried out with any reaction that producesa solid polymer product, typically as a particulate, in the reactionvessel. Example polymerization reactions, the monomers employed, and thepolymers produced, include but are not limited to those described inU.S. Pat. No. 5,679,737 to DeSimone et al. and U.S. Pat. No. 5,780,565to Clough et al. (the disclosures of all patent references cited hereinare to be incorporated herein by reference).

Preferably, the monomer is a vinyl monomer. Examples of vinyl monomersare numerous and include, but are not limited to, aromatic vinylmonomers, conjugated diene monomers, unsaturated acid monomers,nitrogen-based monomers, non-aromatic unsaturated monocarboxylic estermonomers, as well as fluorinated monomers. Mixtures of any of thesemonomers may be employed to allow formation of copolymers, terpolymers,etc.

For the purposes of the invention, the term “aromatic vinyl monomer” isto be broadly interpreted and include, for example, aryl andheterocyclic monomers. Exemplary aromatic vinyl monomers which may beemployed include, for example, styrene and styrene derivatives such asalpha-methyl styrene, p-methyl styrene, vinyl toluene, ethylstyrene,tert-butyl styrene, monochlorostyrene, dichlorostyrene, vinyl benzylchloride, vinyl pyridine, fluorostyrene, alkoxystyrenes (e.g.,paramethoxystyrene), and the like, along with blends and mixturesthereof.

Suitable conjugated diene monomers that may be used include, but are notlimited to, C₄ to C₉ dienes such as, for example, butadiene monomerssuch as 1,3-butadiene, 2-methyl-1,3-butadiene, 2 chloro-1,3-butadiene,and the like. Blends or copolymers of the diene monomers can also beused.

A number of unsaturated acid monomers may be used in the continuouspolymerization. Exemplary monomers of this type include, but are notlimited to, unsaturated mono- or dicarboxylic acid monomers such asacrylic acid, methacrylic acid, itaconic acid, fumaric acid, maleicacid, and the like. Derivatives, blends, and mixtures of the above maybe used.

Nitrogen-containing monomers which may be used include, for example,acrylamide-based monomers may be employed and include, for example,acrylamide, N-methyolacrylamide, N-methyolmethacrylamide,methacrylamide, N-isopropylacrylamide, N-tert-butylacrylamide,N-N′-methylene-bis-acrylamide; alkylated N butoxymethylacrylamide; andnitriles such as acrylonitrile and methacrylonitrile.-methylolacrylamides such as N-methoxymethylacrylamide and N—

Non-aromatic unsaturated monocarboxylic ester monomers may bepolymerized such as, for example, acrylates and methacrylates. Theacrylates and methacrylates may include functional groups such as aminogroups, hydroxy groups, epoxy groups and the like. Exemplary acrylatesand methacrylates include methyl acrylate, methyl methacrylate, ethylacrylate, ethyl methacrylate, butyl acrylate, butyl methacrylate,2-ethylhexyl acrylate, glycidyl acrylate, glycidyl methacrylate,hydroxyethyl acrylate, hydroxyethyl methacrylate, hydroxypropylacrylate, hydroxypropyl methacrylate, isobutyl methacrylate,hydroxybutyl acrylate, hydroxybutyl methacrylate,3-chloro-2-hydroxybutyl methacrylate, n-propyl methacrylate, and thelike. Exemplary amino-functional methacrylates include t-butylaminoethyl methacrylate and dimethylamino ethyl methacrylate. Other monomerssuch as vinyl esters, vinyl halides, and vinylidene halides may also beused.

Exemplary fluoropolymers are formed from monomers which may includefluoroacrylate monomers such as 2-(N-ethylperfluorooctane-sulfonamido)ethyl acrylate (“EtFOSEA”), 2-(N-ethylperfluorooctane-sulfonamido) ethylmethacrylate (“EtFOSEMA”), 2-(N-methylperfluorooctane-sulfonamido) ethylacrylate (“MeFOSEA”), 2-(N-methylperfluorooctane-sulfonamido) ethylmethacrylate (“MeFOSEMA”), 1,1′-dihydroperfluorooctyl acrylate (“FOA”),1,1′-dihydroperfluorooctyl methacrylate (“FOMA”),1,1′,2,2′-tetrahydroperfluoroalkylacrylate,1,1′,2,2′-tetrahydroperfluoroalkyl-methacrylate and otherfluoromethacrylates; fluorostyrene monomers such as α-fluorostyrene and2,4,6-trifluoromethylstyrene; fluoroalkylene oxide monomers such ashexafluoropropylene oxide and perfluorocyclohexane oxide; fluoroolefinssuch as tetrafluoroethylene, vinylidine fluoride, hexafluoropropylene,and chlorotrifluoroethylene; and fluorinated alkyl vinyl ether monomerssuch as perfluoro(propyl vinyl ether) and perfluoro(methyl vinyl ether).

A vast number of copolymers may be formed from any of the abovemonomers, the selection of which is known to one skilled in the art. Inone embodiment, copolymers of maleic anhydride may be formed.Particularly preferred copolymers include, without limitation,styerene/maleic anhydride. Suitable copolymers include, withoutlimitation, fluorinated ethylene propylene copolymer (copolymer oftetrafluoroethylene and hexafluoropropylene), perfluoroalkoxy polymer (acopolymer of tetrafluoroethylene and a perfluoropropylvinylether orperfluoromethylvinylether), sulfur dioxide alternating copolymers suchas those with olefins including, without limitation, butene ornorbornene, and alternating copolymers of ethylene withtetrafluoroethylene. Other preferred copolymers include, withoutlimitation, the following:

-   -   ethylene/propylene/diene monomer    -   ethylene/tetrafluoroethylene    -   vinylidene fluoride/hexafluoropropylene    -   styrene/acrylonitrile    -   acrylonitrile/butadiene/styrene    -   styrene/butadiene    -   styrene/acrylonitrile    -   acrylonitrile/butadiene/styrene    -   styrene/polybutadiene (e.g., high impact polystyrene)    -   ethylene/α-olefins    -   ethylene/propylene/diene monomer    -   ethylene/vinyl acetate    -   ethylene/acrylate monomer/methacrylate monomer    -   vinyl chloride/vinylidene chloride    -   vinyl chloride/vinyl acetate    -   butadiene/acrylonitrile    -   ethylene/tetrafluoroethylene (TFE)    -   tetrafluoroethylene/hexafluoropropylene    -   tetrafluoroethylene/vinyl ether monomer    -   tetrafluoroethylene/functional vinyl ether monomer    -   vinylidene fluoride/tetrafluoroethylene    -   vinylidene fluoride/hexafluoropropylene.

Initiators that may be used in the invention are numerous and known tothose skilled in the art. Examples of initiators are set forth in U.S.Pat. No. 5,506,317 to DeSimone et al., the disclosure of which isincorporated by reference herein in its entirety. Organic free radicalinitiators are preferred and include, but are not limited to, thefollowing: acetylcyclohexanesulfonyl peroxide; diethylperoxydicarbonate; diacetyl peroxydicarbonate; dicyclohexylperoxydicarbonate; di-2-ethylhexyl peroxydicarbonate; tert-butylpemeodecanoate; 2,2′-azobis(methoxy-2,4-dimethylvaleronitrile);tert-butyl perpivalate; dioctanoyl peroxide; dilauroyl peroxide;2,2′-azobis(2,4-dimethylvaleronitrile); tert-butylazo-2-cyanobutane;dibenzoyl peroxide; tert-butyl per-2-ethylhexanoate; tert-butylpermaleate; 2,2-azobis(isobutyronitrile); bis(tert-butylperoxy)cyclohexane; tert-butyl peroxyisopropylcarbonate; tert-butyl peracetate;2,2-bis(tert-butylperoxy) butane; dicumyl peroxide; ditert- amylperoxide; di-tert-butyl peroxide; p-methane hydroperoxide; pinanehydroperoxide; cumene hydroperoxide; and tert-butyl hydroperoxide.Combinations of any of the above initiators can also be used.

Additionally, the invention may accommodate catalyzed reactions such asthose employing, without limitation, transition metal catalystsincluding, for example, iron, nickel, and palladium. If desired, thesecatalysts can be used in combination with ligands such as monodentate,bidentate, or tridentate ligands, the selection of which is known in theart. Examples of such ligands can be found in Ser. No. 09/185,891 filedNov. 4, 1998 and issued as U.S. Pat. No. 6,176,895, the disclosure ofwhich is incorporated herein by reference in its entirety.

Thus, in accordance with the invention, example polymers that may beused in the present invention, and example initiators that may be usedfor such polymers, include but are not limited, those formed from any ofthe above monomers. In one preferred embodiment, vinylidene fluoride(VF2) and acrylic acid (AA) are polymerized, alone or in combination,utilizing diethyl peroxydicarbonate (DEPDC) as the free-radicalinitiator for VF2, and 2,2′-azobis(isobutyronitrile) (AIBN) as thefree-radical initiator for AA. The initiator may be one that providesthe end groups for the polymer chain, and can provide stable end groupsto the polymer if desired. In general, the invention may encompasspolymerizing monomers not limited to those set forth herein, either bythemselves to form homopolymers, or in combination to form, for example,copolymers or terpolymers.

The reaction vessel used to carry out the present invention may be invarious forms or configurations. For example, in one embodiment, thereaction vessel may be a stirred or mechanically agitated reactionvessel, more preferably a stirred reaction vessel that behaves as an“ideal” stirred tank reactor (CSTR), or a continuous loop reactor, morepreferably a continuous loop reactor that behaves as an “ideal” stirredtank reactor. By “ideal stirred tank reactor” is meant one whichsufficiently approximates for commercial conditions a state in which thereactor contents are perfectly mixed so that the system properties areuniform throughout (e.g., for reactor design and analysis purposes). Itshould be understood by one skilled in the art that an ideal stirredtank reactor may encompass physical configurations other than thosedescribed herein. See, e.g., C. Hill, An Introduction to ChemicalEngineering Kinetics and Reactor Design, page 270 (1977). Anotherdefinition of an ideal reactor (e.g., CSTR) is a reactor whosedimensionless exit age distribution function E(Θ), reaches a maximumvalue in the dimensionless time interval between about Θ=0, 0.05, or0.10 and about Θ=0.20, 0.30, and 0.50 and then declines monotonicallyafter reaching the maximum value. In a preferred embodiment of an idealCSTR, the cumulative exit age distribution function, F, has a valuebetween about 0.45 or 0.54 and 0.60 or 0.70 when Θ=1. In a preferredembodiment, the dimensionless exit age distribution for an ideal CSTRreaches its maximum at Θ=0 and has a value of F=0.63 when Θ=1. For thepurposes of the invention, Θ is defined as the actual time divided bythe reactor space time, i.e., the time elapsed in processing one reactorvolume of feed at specified conditions. It should be appreciated thatother embodiments are certainly encompassed within the scope of theinvention. See e.g.,. O. Levenspiel, Chemical Reaction Engineering,3^(rd) Ed., pp. 257-269, John Wiley & Sons, New York, N.Y., (1999).

In a particular embodiment of the invention, an apparatus for thecontinuous polymerization of a monomer in carbon dioxide comprises acontinuous reaction vessel; at least one inlet line connected to thereaction vessel; an effluent line connected to the reaction vessel; aninlet control valve connected to the effluent line; a first separatorand a second separator connected to the inlet control valve, the controlvalve switchable between (i) a first position in which the firstseparator is in fluid communication with the effluent line while thesecond separator is not, and (ii) a second position in which the secondseparator is in fluid communication with the effluent line while thefirst separator is not; and a return line connecting each of the firstand second separators to the reaction vessel so that liquid orsupercritical reaction medium is returned to the reaction vessel fromthe separators while solid polymer is retained in the separator; andcontrol means operatively associated with the return line formaintaining the reaction medium as a liquid or supercritical fluid inthe first and second separators; whereby effluent from the continuousreaction vessel can be (i) continuously passed through the firstseparator while polymer may be removed from the second separator byswitching the inlet control valve to the first position, and (ii)continuously passed through the second separator while polymer may beremoved from the first separator by switching the inlet control valve tothe second position. The separators may be filter separators or cycloneseparators, preferably filter separators. In certain embodiments, asingle cyclone separator normally operates continuously, but a pluralityoperating in parallel can allow for one to be taken off line to becleaned out, etc. Other features as set forth herein may also beincluded.

As alluded to above, a continuous loop reactor can be employed and anembodiment of an example of such a system is depicted as 400 in FIG. 17.An inlet stream 410 comprising liquid or supercritical carbon dioxide,monomer, and initiator passes through an appropriate opening (e.g.,valve, fitting, or the like) 450 and enters the reaction vessel 440. Thereaction vessel comprises inner and outer walls 440 a and 440 brespectively, and in this particular embodiment is in the shape of aloop. Pump 430 facilitates the circulation of the inlet streamthroughout the reaction vessel 440, and ensures that the ingredients arewell mixed. In a preferred embodiment, as long as the flow rate of theinlet stream 410 is small compared to the flow rate of the fluid in theloop of the system, the system properties will not vary significantlyfrom point-to-point in the reaction vessel 440, i.e., will be uniformthroughout. Thus, the performance of the loop reactor will beessentially the same as a stirred or mechanically agitated reactorhaving system properties that do not vary significantly frompoint-to-point throughout the reactor. Preferably, the reaction vessel440 behaves similar to an ideal stirred tank reactor as described indetail herein. As appreciated by one skilled in the art, the reactionvessel 440 may contain mixers, heaters, etc. to enable the ingredientsto be maintained at specified temperature and pressure conditions. Themonomers react in the reaction vessel 440 to form solid polymerparticles. An opening 460 (e.g., a back pressure valve) allows thecontents of the reaction vessel 440 to pass out of the vessel. Resultingeffluent 420 then may be transported to other downstream processingfeatures (e.g., separators and recycling systems) as set forth herein.Additionally, it should be appreciated that the continuous loop reactor400 may include any of the apparatus features described in thespecification, even though these are features may not be depicted inFIG. 17.

FIG. 13 refers to an embodiment in which the separator is present in theform of a plurality of parallel filters, in this example, denoted as 100a and 100 b. Additional filters may be employed as deemed necessary byone skilled in the art. In this embodiment, effluent stream 120 from thecontinuous reaction vessel is passed into one of the filters 100 a or100 b by virtue of the flow being diverted to the desired filter.Polymer is collected in either of the filters and the resulting outgoingstream 130 comprises primarily liquid or supercritical fluid, unreactedinitiator (if any), and unreacted monomer (if any). Stream 130 is thenrepressurized by compressor 110 and the resulting stream 135 is recycledback to the reaction vessel. Although not shown, a purge is preferablypresent between the filters 100 a and 100 b and the compressor 110 tobleed off a portion of the effluent. When a sufficient amount of polymeris collected on the filter such that the pressure drop becomesundesirably high across the filter, the flow is diverted such thatstream 120 passes through the previously-unused filter. Polymer is thencollected from the offline filter. The above procedure may be repeatedas many times as deemed appropriate by one skilled in the art.

FIG. 14 illustrates another embodiment of a separator 200 which may beused in accordance with the invention. Effluent stream 210 containingpolymer, liquid or supercritical fluid, unreacted monomer (if any), andunreacted initiator (if any) enters the separator. During the operationof the separator 200, liquid or supercritical fluid, unreacted monomer(if any), and unreacted initiator (if any) passes through pores 240 inthe inside walls 260 of the separator. The walls 260 may be formed froma variety of materials that are porous including, without limitation,sintered metal, ceramic, etc. Upon passing through the walls 260, thefluid stream enters chamber 270 and leaves this chamber through exitline 280. The stream leaving through 280 may be disposed of as deemedappropriate. As an example, the stream may be recycled to the reactionvessel. Advantageously, the size of the pores are such that polymer doesnot pass through, but instead collects on the inside surface of thewalls 260.

A rotating device 220 with drive 225 may be present in variousconfigurations is present in the separator and serves to continuouslyremove the polymer that collects on the inside surface of the walls 260.In this embodiment, the rotating device 220 is present in the form of ascrew, although other types of devices can be employed within the scopeof the invention. The screw 220 removes the polymer from the wall 260and conveys the polymer through the bottom 250 of the separator. A solidwall 290 surrounds the bottom of the screw as depicted in FIG. 14. Inorder to minimize loss of reaction medium and unreacted monomer andinitiator through bottom 250, the screw 220 is designed to melt thepolymer and form a seal in the screw 220. The molten polymer is conveyedthrough exit 250 from the high pressure region of the device to anessentially ambient pressure region, where it is cooled and processed bytechniques known to one skilled in the art. Thus, the separator 200 mayoperate in a continuous fashion.

A preferred embodiment for a cyclone-type separator 300 is depicted inFIG. 15. In this embodiment, cyclone 310 is in fluid communication withparallel filters 320 a and 320 b positioned downstream of the cyclone310. Incoming effluent stream 330 containing liquid or supercriticalfluid, unreacted monomer (if any), and unreacted initiator (if any)enters the cyclone 310 which results in the formation of a bottom stream340 containing a relatively high percentage of polymer and a top stream350 containing primarily liquid or supercritical fluid, unreactedmonomer (if any), and unreacted initiator (if any), along with polymer.The top stream 350 is diverted to either of filters 320 a or 320 b in amanner described hereinabove (e.g., see FIG. 13). This arrangementallows for the removal of polymer from stream 350 such that theresulting exit stream 360 contains a sufficiently low level of polymersuch that it is suitable for recycle to the reaction vessel, if sodesired. Polymer may then be collected from any of the filters 320 a and320 b using appropriate techniques.

In one embodiment of the invention, the method for carrying out thecontinuous polymerization of a monomer in carbon dioxide comprises thesteps of: (a) providing an apparatus including a continuous reactionvessel, a first separator, and a second separator; (b) carrying out apolymerization reaction in the reaction vessel by combining a monomer,an initiator, and a carbon dioxide reaction medium therein, wherein thereaction medium is a liquid or supercritical fluid, and wherein thereaction produces a solid polymer product in the reaction vessel; then(c) withdrawing a continuous effluent stream from the reaction vesselduring the polymerization reaction, passing at least a portion of theeffluent stream through the first separator while maintaining theeffluent stream as a liquid or supercritical fluid and separating thesolid polymer therefrom; and then returning at least a portion effluentstream to the reaction vessel; and then (d) withdrawing a portion of thecontinuous effluent stream from the reaction vessel during thepolymerization reaction, passing the effluent stream through the secondseparator while maintaining the effluent stream as a liquid orsupercritical fluid and separating the solid polymer therefrom, and thenreturning at least a portion of the effluent stream to the reactionvessel, while concurrently removing the solid polymer separated in thefirst separator during the withdrawing step (c). Preferably, step (d) isfollowed by the step of: (e) repeating the withdrawing step (c) whileconcurrently removing the solid polymer separated in the secondseparator during the withdrawing step (d). Preferably, an initiator isemployed in step (b). Preferably, a purge is located in a returnpolymerization line between the separator(s) and the reaction vessel soas to remove an amount of the effluent stream as deemed appropriate byone skilled in the art. In these preferred embodiments, the at least aportion of effluent that is returned to the reaction vessel is typicallya fraction less than one.

Any suitable system or apparatus may be used as the control means formaintaining said reaction medium as a liquid or supercritical fluid inthe separator and the return line (and preferably at a pressure not morethan 50 or 100 psi different from the pressure in said reaction vesselduring polymerization of monomer therein). Examples include, but are notlimited to, charging fluid into the system wherein the charging may becontrolled through the use of a computer which may be analog or digital,removing reaction medium from the system wherein the removing may becontrolled through the use of a computer which may be analog or digital,adding heat to the system, wherein adding the heat may be controlledthrough the use of a computer which may be analog or digital, removingheat from the system, wherein removing the heat may be controlledthrough the use of a computer which may be analog or digital, or pumpingthe reaction medium, wherein pumping may be controlled through the useof a computer which may be analog or digital.

The present invention is explained in greater detail in the followingnon-limiting Examples.

EXAMPLE 1 Chain-Growth Precipitation Polymerizations

The experimental system consists of an intensely mixed, continuousstirred tank reactor (CSTR), followed by two high-pressure filters inparallel, where the polymer is collected. This method is widelyapplicable to various monomers in heterogeneous polymerizations, bothwith and without surfactants. Herein we report on our experiments withvinylidene fluoride (VF2) performed at a temperature of 75° C., apressure of 275 bar, and at residence times from 15 to 50 minutes. Thepoly(vinylidene fluoride) polymer (PVDF) was collected as a dry“free-flowing” powder, and has been characterized by gel permeationchromatography (GPC).

Materials. VF2 monomer was donated by Solvay Research, Belgium andSFE/SFC grade CO₂ was donated by Air Products & Chemicals, Inc. Allother chemicals were obtained from Aldrich Chemical Company.

Initiator Synthesis. The DEPDC (diethylperoxydicarbonate) initiator wassynthesized as previously reported, using water as a reaction medium andextracting the initiator into Freon 113 (Mageli, O. L.; Sheppard, C. S.;In Organic Peroxides, Vol. I, Swern D, Eds.; Wiley-Interscience, NewYork, 1970 pp. 1-104; Hiatt, R. In Organic Peroxides, Vol. II, Swern D,Eds.; Wiley-Interscience, New York, 1970 pp. 799-929; Strain, F.;Bissinger, W. E.; Dial, W. R.; Rudolf, H.; DeWitt, B. J.; Stevens, H.C.; Langston, J. H. J. Am. Chem. Soc. 1950, 72, 1254-1263). Allmanipulations of the initiator were performed in an ice bath and thefinal product was stored in a cold chest at −20° C. The iodine titrationtechnique, ASTM Method E 298-91, was utilized to determine theconcentration of active peroxide in the solution.

Polymerization Apparatus. A schematic of the equipment used for thepolymerization is shown in FIG. 1. Carbon dioxide 14 and monomer 15 arepumped continuously by Isco syringe pumps 16 and 17 in constant flowmode and mixed by an 8-element static mixer 8, before entering thereactor 18. The initiator solution is also pumped continuously by anIsco syringe pump 19 in constant flow mode, and enters the reactor 18 asa separate stream. All feed lines have check-valves to preventback-flow, thermocouples, and rupture disks for safety in case of overpressurization. The CSTR is an 800 mL Autoclave Engineers (AE) autoclavewith a magnedrive to provide mixing of ingredients with an AEdispersimax impeller. The reactor is heated by a furnace, has aninstalled pressure transducer (Druck) and a thermowell containing athermocouple (Omega Engineering). FIG. 1 depicts a continuous stirredtank reactor 18. It should be appreciated that other reactors can beemployed in the system depicted in FIG. 1 such as, without limitation, acontinuous loop reactor as referred to herein.

The effluent stream leaves the CSTR 18 through the bottom, and isdirected by a 3-way ball-valve 10 (HIP) to one of two 280 mL filterhousings (Headline) containing 1 μm filters where the solid polymer iscollected. Unreacted monomer, initiator and CO₂ pass through the filtersand flow through a heated control valve 12 (Badger). This control valvefunctions as a back-pressure regulator, which controls the reactorpressure at the desired set-point. The effluent stream passes through awater bath to remove unreacted peroxide, while the gaseous CO₂ andmonomer is safely vented into a fume-hood. Very low levels of polymerwere found in the water bath, so essentially all precipitated polymerwas collected on the 1 μm filters.

The entire polymerization apparatus 20 was computer controlled andmonitored. The supervisory control and data-acquisition (SCADA) systemconsists of National Instruments BridgeVIEW software and Fieldpointinput/output modules. Input modules were utilized for reading pressuretransducers and thermocouples. Output modules were utilized to controlthe reactor furnace, and the control valve. All control functions wereperformed utilizing PID algorithms.

Polymerization Procedure. The reactor was first heated to the desiredtemperature and the agitator was set to 1800 revolutions per minute(RPM). The system was then purged with N₂. After about 2 hours, thecontrol valve was closed and the system was pressurized with CO₂ to thedesired reactor pressure. The desired CO₂ flow rate was set and thetemperature and pressure of the system were allowed to stabilize.Temperature control was ±0.2° C., while pressure control was ±1 bar.When the system had stabilized, the initiator flowrate was set andinitiator was allowed to flow through the system for at least 3residence times in order to remove impurities. Monomer flow was thenstarted. At least 5 residence times after the introduction of monomer,with the CSTR at steady-state, the 3-way ball valve was switched and thestream exiting the CSTR was fed to the empty filter/collector, wheresteady-state polymer was collected for between 30 and 60 minutes. Afterthis time, the ball valve was switched so that effluent flowed to theoriginal collector, and the monomer and initiator feed streams werestopped so that only pure CO₂ was fed through the system for cleaning.The system was finally vented and the polymer collected and weighed.

Results and Discussion. We have developed and demonstrated a continuous,once through system for precipitation polymerizations in scCO₂, as shownin FIG. 1. Table 1 shows the reactor conditions for several experimentswith VF2 polymerization initiated by DEPDC. Table 2 provides thepolymerization results and GPC data for the poly(vinylidene fluoride)(PVDF) polymer produced in these experiments. The conversion of VF2 inthese polymerizations (Conversion=moles of monomer reacted/moles ofmonomer fed) ranged from 7 to 24%. Unlike a batch polymerization, highconversions are not required for a continuous system, as the monomer isrecycled. The rate of polymerization (R_(p)) for the CSTR system reacheda maximum of 19×10⁻⁵ mol/L·s, at a feed monomer concentration of 1.7mol/L. This rate will increase as the concentration of monomer isincreased. In the batch polymerization of VF2 in scCO₂, the averageR_(p) at 3.0 M monomer concentration, using an acyl peroxide initiatorat 65° C., was 0.2×10⁻⁵ mol/(L·s) (Kipp, B. PhD Thesis, University ofNorth Carolina-Chapel Hill. 1998).

TABLE 1 Reactor Conditions for Polymerization of Vinylidene Fluoride.Pres- Run sure Temp τ* M_(CO2)** m_(Initiator)** m_(VF2)** [VF2]_(INLET)# (bar) (° C.) (min) (g/min) (mg/min) (g/min) (mol/L) 1 276 75 21 26.44.9 1.9 0.77 2 276 75 28 19.9 14.7 1.4 0.77 3 276 75 21 26.5 19.5 1.90.77 4 276 75 21 26.5 32 1.9 0.77 5 276 75 14 39.8 29.2 2.0 0.77 6 27675 20 26.5 19.6 3.8 1.45 7 276 75 22 26.5 19.7 0.94 0.40 *τ = reactorresidence time = reactor volume/total volumetric flow rate **m = massflow rate

TABLE 2 Polymerization Results and Polymer Characterization Data forPVDF. R_(p) Run X (mol/L · s) M_(n) M_(W) # (%)* (×10⁵)** (×10⁻³)(×10⁻³) M_(W)/M_(n) 1 6.9 4.0 20 33 1.7 2 18.4 7.9 15 21 1.4 3 18 10.314 21 1.5 4 24 13.7 12 17 1.4 5 11 9.5 18 45 2.5 6 16 16.7 29 60 2.1 720 5.9 10.5 15 1.4 *Conversion (X) was determined gravimetrically fromsteady-state polymer collection. **Rate of polymerization (R_(p)) wascalculated by R_(p) = ([VF2]_(INLET) − [VF2]_(OUTLET))/τ

The GPC results indicated that the molecular weight distributions (MWDs)were unimodal.

Conclusions. This example describes a system for the continuouspolymerization of various monomers in scCO₂. The feasibility of thecontinuous precipitation polymerization of VF2 and AA has beendemonstrated using an intensely-agitated, continuous stirred tankreactor (CSTR). Rates of polymerization of VF2 in the CSTR aresignificantly higher than the average rates of batch polymerization,under similar conditions.

EXAMPLE 2 Continuous Precipitation Polymerization of Vinylidene Fluoridein Supercritical Carbon Dioxide: Comparison of Experimental to ModelR_(p)

This example describes the heterogeneous polymerization of vinylidenefluoride. Poly(vinylidene fluoride) (PVDF) is a semicrystalline polymerand is produced commercially by either emulsion or suspension batchtechniques at polymerization conditions of between 10-200 bar attemperatures from 10-130° C. (Dohany, J. E. and J. S. Humphrey,Vinylidene Fluoride Polymers, in Encyclopedia of Polymer Science andEngineering, H. F. Mark, et al, Editors. 1989, Wiley: New York. p.532-548; Russo, S., M. Pianca, and G. Moggi, Poly(vinylidenefluoride),in Polymeric Materials Encyclopedia, J. C. Salamone, Editor. 1996, CRC:Boca Raton. p. 7123-7127). The emulsion technique requires that thefinal polymer latex be first coagulated, thoroughly washed, thenspray-dried before a free-flowing powder is obtained. The suspensiontechnique requires separation of the polymer from the water phase,thorough washing, then drying. Vinylidene fluoride monomer normallycontains no inhibitors and PVDF polymer does not require additives forstabilization during melt-processing, thereby qualifying this polymerfor applications such as ultrapure water systems where high puritymaterials are required. Due to the inherent disadvantages of thetraditional techniques for preparing PVDF, such as additives requiredfor polymerization and difficult to treat waste streams, a continuousenvironmentally-friendly process is attractive.

This example also describes the kinetics and mechanism of VF2polymerization initiated by the organic peroxide, diethylperoxydicarbonate (DEPDC), using the novel highly agitated continuoussystem of the present invention. Not intending to be bound by theory,the information gained in this work is useful for developing predictivekinetic models that can describe the rate of polymerization (R_(p)) andmolecular weight distribution (MWD) for experimental conditions ofinterest. It should be noted that until now, there has been nosystematic investigation of the kinetics of free-radical polymerizationscarried out in CO₂, either in batch, or using a CSTR. As well, verylittle polymerization data is present in the literature on the PVDFsystem in particular, and fluorinated monomers in general. We reporthere on experiments that have been performed at stirring rates from1300-2700 rpm, initiator inlet concentrations ranging from 8-50 (×10⁻⁴)M, monomer inlet concentrations ranging from 0.4-3.5 M, temperaturesranging from 65-80° C. (at constant CO₂ densities of 0.74 g/ml), andresidence times from 10 to 50 minutes. The polymer was collected in allcases as a dry “free-flowing” powder, and has been characterized by gelpermeation chromatography (GPC).

1. Materials and Methods.

Materials. VF2 monomer was provided by Solvay Research, Belgium andSFE/SFC grade CO₂ was provided by Air Products & Chemicals, Inc. Allother chemicals were obtained from Aldrich Chemical Company.

Initiator Synthesis. The DEPDC initiator was synthesized as previouslyreported, using water as the reaction medium and extracting theinitiator into Freon 113 (HPLC Grade) (Mageli, O. L. and C. S. Sheppard,Organic Peroxides, ed. D. Swern. Vol. I. 1970, New York:Wiley-Interscience. 1-104.; Hiatt, R., Organic Peroxides, ed. Swern D.Vol. II. 1970, New York: Wiley-Interscience. 799-929). All manipulationsof the initiator were performed in an ice bath and the final product wasstored in a cold chest at −20° C. The iodine titration technique, ASTMMethod E 298-91, was utilized to determine the concentration of activeperoxide in the solution.

Polymerization Apparatus. The equipment and the polymerization procedureis described in Example 1 above. Modifications to the system for theinstant example include: a) replacing the reactor furnace with atemperature jacket (Autoclave Engineers) through which heating/coolingfluid is circulated to provide superior temperature control for thereactor; b) addition of a gas chromatograph (SRI 8610C) which samplesthe exit stream (after filtration) directly through a HPLC valve(Valco). The GC column is a silica column while the oven temperature wasisothermal at 55° C; and c) addition of a counter-current heat exchangeron the effluent line of the CSTR to cool the exiting polymer stream toambient temperature.

GPC. All gel permeation chromatography (GPC) measurements of the PVDFpolymer samples were performed on a Waters-Alliance HPLC system with 2×HR5E and 1× HR2E columns using N,N-Dimethylformamide (DMF) modified withLiBr 0.1M. The following conditions were adopted: 1) column compartmenttemperature at 40° C., 2) flow rate of mobile phase, 1 ml/min. 3) sampleinjection volume, 100 μl, 4) no sample filtering 5) sample concentrationof 0.1 wt % in mobile phase (samples are conditioned in mobile phase at60° C. for one hour but can be injected at room temperature).Calibration of the GPC was performed at 40° C. directly with acalibration curve obtained using narrow MWD PMMA standards purchasedfrom Polymer Laboratories Ltd. The Mark-Houwink constants for theuniversal calibration curve were K=1.32 10⁻⁴, a=0.674 for PMMA andK=1.14 10⁻⁵, a=0.97 for PVDF.

Polymerization Control. Polymerization takes place in our continuoussystem in a highly-agitated CSTR, where CO₂, VF2 and DEPDC arecontinuously fed to the reactor and mixed under isothermal conditions,while the produced heterogeneous polymer, i.e. PVDF, as well as CO₂ andunreacted VF2 and DEPDC, continuously leave the reactor. No recycle ispresently implemented (both for simplicity and to prevent the buildup ofimpurities). Control of the reactor temperature (T) and pressure (P) wasexcellent during a polymerization, varying within very close tolerances(T=±0.2° C. and P=±1 bar). Feed rates of initiator and monomer from thesyringe pumps are ±0.1%.

Attainment of Steady-State. In order to determine the attainment ofsteady-state (SS), both gas-chromatograph (GC) analysis and varying thepolymer collection time was used. The CO₂ and VF2 peaks could beseparated by GC. Calibration of the GC was performed using CO₂ and VF2flowrates determined by the syringe pumps. Densities of VF2 and CO₂ inthe cooled syringe pumps (cooling the syringe pumps by chillercirculators allows for easier condensing of liquified gases) and heatedreactor were determined from data provided by Solvay for VF2(Peng-Robinson equation of state) while CO₂ densities were determinedfrom US National Institute of Standards and Technology (NIST) data.

2. Results and Discussion

Attainment of Steady-State. FIG. 2 shows a GC analysis used to determinethe attainment of steady-state for a typical polymerization run. In thisfigure the effluent VF2 concentration is measured as a function of time,in units of the reactor residence time, τ. For a typical polymerizationrun, steady-state was attained after about 5 τ. Polymer collection wasnormally initiated after 5 τ's by switching to the SS collector. AfterSS polymer collection was complete, the exit stream was turned back tothe non-SS filter such that SS polymer was not mixed with non-SSpolymer. After the reactor had been on stream for at least 5 τ's,collection of polymer for varying time lengths was found to giveidentical polymer weight/collection time ratios, confirming the resultsfrom the GC analysis.

Phase Behavior. Under the experimental conditions studied, the monomer,VF2 (HFC-1132a), and the free-radical initiator, DEPDC, were found to bemiscible with CO₂ while the formed polymer powder, PVDF, is imiscible inCO₂ or in VF2, for all experimental ranges studied (by off-line studiesin a high-pressure view cell)(Lora, M., J. S. Lim, and M. A. McHugh,Comparison of the solubility of PVF and PVDF in Supercritical CH2F2 andCO2 and in CO2 with Acetone, Dimethyl Ether, and Ethanol. J. Phys. Chem.B., 1999. 103(14): p. 2818-2822). This phase behavior defines aprecipitation polymerization. As mentioned in Example 1, all formedpolymer powder was collected by the 1 μm filters, with very low levelsof polymer being found in the exit water bath. After each reaction, theinside of the reactor was normally very lightly coated with powder. Thethin powder layer was always dry, not tacky, and could easily be wipedoff the reactor walls. No sticky film formation was observed and thewall temperature of the reactor never exceeded the melting point of thepolymer. Under these experimental conditions, no evidence was obtainedthat the powder was building up on the reactor walls, or in the reactor,as the powder layer was always very thin and experiments varying thecollection time of the steady-state stream gave identical polymerweight/collection time ratios.

RTD and Initiator Decomposition Studies. The residence time distribution(RTD) of the experimental reactor was determined as well as thedecomposition kinetics of the DEPDC free-radical initiator. For allconditions studied, the RTD of the reactor was found to model that of anideal CSTR. These experiments were performed in pure CO₂, under typicalexperimental conditions of T and P, without the presence of any polymerpowder.

Table 3 provides the initiator decomposition rate constants for DEPDC inscCO₂. It should be noted that no significant solvent dependence wasobserved for decomposition of DEPDC in scCO₂ compared to the literaturevalue that used radical scavenging 2,2′-Oxydiethylene bis(allylcarbonate) as solvent (Strain, F., et al., Esters of PeroxycarbonicAcids. J. Amer. Chem. Soc., 1950. 72: p. 1254-1263). The initiatorefficiency found, f=0.6, is also very typical for an organic peroxide(Hamielec, A. E. and H. Tobita, Polymerization Processes. Ullmann'sEncyclopedia of Industrial Chemistry. 1992. 331). For the kineticanalysis of PVDF polymerization presented in this paper, the k_(d)'sfrom Table 3 were used, while f=0.6 was used for the initiatorefficiency for all temperatures studied.

TABLE 3 Initiator Decomposition Rate Constants Temperature (° C.) k_(D)(×10⁴ s) F 65 2.4 0.61 70 4.3 0.69 75 10.3 0.59 85 35.1 0.63

Effect of Agitation on VF2 Polymerization. Our first polymerizationexperiments dealt with the effect of agitation on the polymerization.FIG. 3 provides the effect of stirring rate and agitator type on monomerconversion (X). The 1.25″ diameter dispersimax™ agitator, which is a6-bladed Rushton-type turbine (d/D=0.42), was studied from 1300-2700rpm. This type of agitator provides mainly radial flow (Geankoplis, C.J., Transport Processes and Unit Operations. Third ed. 1993, EnglewoodCliffs, N.J.: Prentice Hall). It is clear that the conversion is notaffected by the stirring rate for the region investigated. For thelowest stirring rate investigated, 1300 rpm, an in-house designedpitched-blade turbine agitator also was investigated. This agitator is a4-bladed, 45° pitch, upward pumping agitator designed to provide acombination of axial and radial flow in order to suspend precipitatedparticles. This agitator was studied at the lowest rpm to minimizebearing wear in case of any offset in manufacture. The conversionobtained with this agitator is identical to that obtained with theDispersimax™ impeller, indicating that no effect of agitator geometrywas obtained on conversion, for the conditions studied. In addition tothe conversions being identical for the mixing study, PVDF molecularweights (MWs) determined by gel permeation chromatography (GPC), werefound to be identical for polymer samples taken at both the lowest RPMstudied for both agitators, and the highest RPM studied for theDispersimax™ impeller. The results from the X and MW data lead us tobelieve that the kinetics were not effected by mixing in this study. Forall subsequent experiments reported, the DispersiMax™ impeller was usedat a stirring rate of 1800 rpm.

Determination of the Rate of Polymerization (R_(p)) Model Equation. (i)Determination of Monomer Order. In order to derive a model equation forthe rate of polymerization (R_(p)), we must first determine the order ofthe reaction with respect to both monomer and initiator. The massbalance for monomer around the reactor, modeled as an ideal CSTR, can besimplified to provide the rate of polymerization (R_(p)):

$\begin{matrix}{R_{P} = \frac{\left( {\lbrack M\rbrack_{IN} - \lbrack M\rbrack_{OUT}} \right)}{\tau}} & (2)\end{matrix}$

For an ideal CSTR, the reactor concentrations are the same as the outletconcentrations (Levenspiel, O., Chemical Reaction Engineering. Seconded. 1972, New York: John Wiley & Sons). For the work reported on here,the outlet monomer concentration was determined by mass-balance(gravimetrically by weighing the polymer collected at steady-state) andconfirmed by on-line GC analysis. This allows us to determine R_(p)experimentally, as both the inlet monomer concentration and the meanresidence time of the reactor τ are known. FIG. 4 provides the plot ofR_(p) versus [VF2]^(1.0) which illustrates that this polymerization isfirst-order with respect to monomer. First-order dependency is generallyobtained in free-radical kinetics for monomer consumption (Odian, G.,Principles of Polymerization. 3rd ed. 1991, New York: John Wiley & Sons,Inc).

For subsequent experiments, monomer inlet concentrations of 0.82 M wereused. Equation 2 is used for the experimentally determined R_(p)'sreported on here.

ii) Determination of Initiator Order. The initiator concentration in thereactor, which is identical to the outlet concentration for an idealCSTR, is given by:

$\begin{matrix}{\lbrack I\rbrack_{OUT} = \frac{\lbrack I\rbrack_{IN}}{1 + {k_{D}\tau}}} & (7)\end{matrix}$Hence, the concentration of initiator in the reactor is given by theoutlet concentration, [I]_(OUT), which can be determined from the inletconcentration, [I]_(IN), the mean residence time, τ, and thedecomposition rate constant, k_(D) (provided in Table 3). FIG. 5provides the plot of monomer conversion (X) versus the inlet initiatorconcentration, i.e. [I]_(IN). It is evident that the conversionincreases with an increase in initiator concentration, as more freeradicals are generated to initiate polymer chains.

FIG. 6 provides the plot of R_(p) versus [I]_(OUT) ^(0.5), which showsthat the order of the reaction with respect to initiator is 0.5.However, note that a small offset error occurred, [I−I*]^(0.5) Forfuture calculations, [I−I*]^(0.5), is used to account for this error.For subsequent experiments, initiator inlet concentrations of 3 mM wereused.

Half-order dependency is normally obtained in free-radical kinetics forinitiator consumption, although conventional heterogeneouspolymerizations, such as vinyl chloride or acyrlonitrilepolymerizations, often show initiator exponents exceeding this classicalvalue (Eastrnond, G. C., Radical Polymerization, in Encyclopedia ofPolymer Science and Engineering, H. Mark, Editor. p. 708-855). Thisbehavior is often attributed to the polymer radicals precipitatingduring the reaction in the nonsolvent environment, forming tightlycoiled chains which “trap” or “occlude” the radicals. These trappedradicals can react with monomer but have trouble terminating, henceleading to auto acceleration and initiator exponents greater than 0.5.Normally radical trapping decreases with increasing polymerizationtemperatures. As we used a relatively high temperature in this study,i.e. 75° C., and CO₂ densities that cause the polymer chains to beplasticized, hence increasing the free-volume of the polymer andmobility of the chain-ends, radical trapping is minimized.

iii) Determination of the k_(p)/k_(t) ^(0.5) Ratio. To continue our goalfor determining an appropriate model equation for the R_(p) of VF2 inour experimental system in scCO₂, and assuming simple chain-growthkinetics in a CSTR, we can develop our model using the following mainassumptions, (1) Polymerization in fluid phase only. (2) QSSA, LCA:Quasi-steady-state assumption (QSSA) is considered for the radicalspecies. Moreover, because the large molecular weights usually obtained,the long-chain assumption (LCA) is introduced, thus neglecting anydependence of reactivity upon length. i.e.:R _(p) =k _(p) [M·IM] _(OUT)  (8)AndR _(i)=2k _(t) [M·] ² =R _(i)  (9)

Combining (8) and (9) givesR _(p) =k _(p) [M] _(OUT)(R _(i)/2k _(t))²  (10)and as:R _(i)=2fk _(d) [I] _(OUT)  (11)we can derive an expression for the theoretical R_(p) for ourexperimental reactor:R _(p)=(k _(p) /k _(t) ^(0.5))(fk _(d)([I] _(OUT) −I*)^(0.5) [M] _(OUT)^(1.0)  (12)Equation 12 is hereafter referred to as the model equation fordetermining the R_(p). In order to utilize our model equation 12, wemust first determine experimental values for the k_(p)/k_(t) ^(0.5)ratio, which should only depend on the reactor temperature for a givenCO₂ density (the polarity of the solvent as effected by monomerconcentration may also have an effect on this ratio). In order to studythe effect of the reaction temperature on our polymerizations, henceallowing us to determine the k_(p)/k_(t) ^(0.5) ratio as a function oftemperature, the reactor pressure was varied to provide a constant CO₂density of 0.74 g/ml. Combining equations (12) and (2) provides anexpression that allows k_(p)/k_(t) ^(0.5) to be determinedexperimentally.k _(p) /k _(t) ^(0.5) ={[M] _(IN) −[M] _(OUT)}/((τ{fk _(d)([I] _(OUT)−I*)}^(0.5) [M] _(OUT) ^(1.0))   (13)

This allows us to utilize the Arrhenius relationship for the k_(p)/k_(t)^(0.5) ratio.

$\begin{matrix}{{\ln\left\lbrack {k_{p}/k_{t}^{0.5}} \right\rbrack} = {{\ln\left\lbrack {A_{p}/A_{t}^{0.5}} \right\rbrack} - \frac{E_{p} - \left( {E_{t}/2} \right)}{RT}}} & (14)\end{matrix}$

FIG. 7 provides the plot of equation 14, which is linear indicating thatthis system follows the Arrhenius relationship in the regioninvestigated. An E_(p)−(E_(t)/2) value of 69 kj/mol was determined fromthis plot. Table 4 provides the k_(p)/k_(t) ^(0.5) values determined forthe 4 temperatures studied.

FIG. 8 shows the effect of reactor temperature on the experimentallydetermined R_(p) (determined from equation 2) and compares these valuesto those predicted from model equation 12. The agreement with the modelequation is excellent. As expected, R_(p) increases rapidly withtemperature.

TABLE 4 k_(p)/k_(t) ^(0.5) Values determined from Experimental DataTemperature (° C.) K_(p)/k_(t) ^(0.5) 65 0.12 70 0.18 75 0.25 80 0.36

Effect of Reactor Mean Residence Time (τ) on R_(p). In order to test ourdeveloped model under varying experimental conditions, the effect of themean residence time, τ, as controlled by the flow of reactants wasinvestigated in the region of 10-50 minutes. The flow-rates of CO₂,monomer and initiator were adjusted for each of the τ values studied togive identical inlet concentrations of monomer and initiator. FIG. 9provides the R_(p) values determined experimentally from equation 2, andcompares them to those calculated from our model equation 12. The R_(p)values decrease with increasing τ, as expected, as low τ values have thehighest [VF2]_(OUT) and [I]_(OUT) values. Once again, the experimentaldata follows the model equation very closely.

FIG. 10 provides the parity plot of all experimental data reported on inthis study, which compares the experimental R_(p) data determined fromequation 2, to that predicted from model equation 12. This plot providesstrong evidence that the R_(p)'s can be described quite well by oursimple model equation 12. This behavior indicates that we can describeour precipitation polymerization as a pseudobulk system.

Determination of Model Equations Describing the MWDs of PVDF. Our modelequation for M_(n) is obtained from the kinetic chain length, ν, byassuming that: a) the heterogeneous polymerization occurs in a singlephase, b) there is no chain transfer, c) all termination occurs bycombination, and d) the accumulated polymer distribution is the same asthe instantaneous distribution (which is true for an ideal CSTR). Inother words, the instantaneous MWD defines a most probable distributionwith polydispersity index of 1.5 where all termination occurs bycombination (Flory, P. J., Principles of Polymer Chemistry. 1953,Ithaca, N. Y.: Cornell University Press. 161).

$\begin{matrix}{M_{n} = \frac{M_{o}{k_{p}\lbrack M\rbrack}_{OUT}}{\left( {{f \cdot k_{d}}k_{t}} \right)^{1/2}\left( {I - I^{*}} \right)^{1/2}}} & (15) \\{M_{W} = {1.5 \cdot M_{n}}} & (16)\end{matrix}$

FIG. 11 shows how the number and weight average molecular weights (M_(n)and M_(W)) increase with increasing outlet monomer concentration andprovides a comparison to model equations. We see that our experimentaldata fits the simple model equations reasonably well at low monomerconcentrations for M_(n) and fits very well for M_(W) across all monomerconcentrations studied.

Conclusions. For the precipitation polymerization of VF2 in scCO₂,simple chain-growth kinetics were approximated for this heterogeneouspolymerization and the order of the reaction with respect to initiatorwas found to be 0.5 and with respect to monomer 1.0. Stirring rate andagitator design were found to have no effect on the rate ofpolymerization. The conversion of VF2 in these polymerizations rangedfrom 7 to 26%, and the rate of polymerization (R_(p)) reached a maximumof 34×10⁻⁵ mol/L·s at a VF2 feed monomer concentration of 3.5 mol/L at75° C. The poly(vinylidene fluoride) (PVDF) polymer was collected as adry “free-flowing” powder, and has been characterized by gel permeationchromatography (QPC) giving M_(w)'s up to 104 kg/mol and PDIs as low as1.3. Termination of polymer chains appears to occur by combination.

EXAMPLE 3 Large-Scale Apparatus

One embodiment of a scaled-up apparatus for implementing the presentinvention is described in FIG. 12. Initiator is transported continuouslyto a reactor, along with, and at a separate location from, carbondioxide and monomer, which are introduced via a recirculation linethrough a recirculation pump. Make-up CO₂ and monomer may be introducedthrough the top of the reactor if so desired as depicted. Upon exitingthe reactor, the effluent stream is cooled and is transported to aseparator (e.g., a filter or cylcone configuration). Polymer productexits through the bottom of the reactor and is collected in theseparator. Carbon dioxide, either through the top or bottom of thereactor, may be employed to assist with transporting the polymer to alow pressure bag filter or extruder hopper.

An effluent stream comprising carbon dioxide, unreacted monomer (ifany), and unreacted initiator (if any) is recycled back to the reactoras shown in FIG. 12. A purge is bled off from as a portion of theeffluent stream. A liquid stream may optionally be collected through thebottom of the condenser if so desired.

EXAMPLE 4 Residence Time Distribution of an Experimental Reactor

The residence time distribution of an experimental reactor of thepresent invention was evaluated using a pulse injection of tracer. Fortemperatures between 50° C. and 90° C. and pressures between 207 bar and320 bar, and mean residence times as low as 13 minutes, the experimentalreactor behaved as an ideal CSTR. The results are presented in FIG. 16.

In the drawings and specification, there have been disclosed typicalpreferred embodiments of the invention and, although specific terms areemployed, they are used in generic and descriptive sense only and notfor the purposes of limitation, the scope of the invention being setforth in the following claims.

1. An apparatus for the continuous polymerization of a monomer in carbondioxide; said apparatus comprising: a continuous reaction vessel; aneffluent line connected to said reaction vessel; a separator connectedto said effluent line; a return line connecting said separator to saidreaction vessel so that liquid or supercritical reaction medium isreturned to said reaction vessel from said separator while solid polymeris retained in said separator; and control means for maintaining saidreaction medium as a liquid or supercritical fluid in said separator andsaid return line and at a pressure not more than 100 psi different fromthe pressure in said reaction vessel during polymerization of monomertherein.
 2. An apparatus according to claim 1, wherein said separator isa filter.
 3. An apparatus according to claim 1, wherein said separatorcomprises a plurality of filters in parallel with each other.
 4. Anapparatus according to claim 1, wherein said separator comprises arotating device therein.
 5. An apparatus according to claim 1, whereinsaid separatoris a cyclone separator.
 6. An apparatus according to claim5, further comprising a filter positioned downstream of and in fluidcommunication with said cyclone separator.
 7. An apparatus according toclaim 1, further comprising a cooler positioned on said effluent linebetween said reaction vessel and said control valve.
 8. An apparatusaccording to claim 1, further comprising a recirculation pump positionedon said return line between said separator and said reaction vessel. 9.An apparatus according to claim 1, further comprising a condenserpositioned on said return line between said separator and said reactionvessel.
 10. An apparatus according to claim 1, wherein said reactionvessel is a stirred tank reactor.
 11. An apparatus according to claim 1,wherein said reaction vessel is an ideal stirred tank reactor.
 12. Anapparatus according to claim 1, wherein said reaction vessel is acontinuous loop reactor.
 13. An apparatus according to claim 1, whereinsaid reaction vessel is configured to provide a dimensionless exit agedistribution function (E(Θ)) which reaches a maximum value between aboutΘ=0 and about Θ=0.3 and thereafter declines monotonically after reachingits maximum value.
 14. An apparatus according to claim 1, wherein saidreaction vessel is configured to provide a cumulative exit agedistribution (F) of from about 0.45 to about 0.70 when Θ=1.
 15. Anapparatus for the continuous polymerization of a monomer in carbondioxide; said apparatus comprising: a continuous reaction vessel; aninlet line connected to the reaction vessel; an effluent line connectedto said reaction vessel; an inlet control valve connected to saideffluent line; a first separator and a second separator connected tosaid inlet control valve, said control valve switchable between (i) afirst position in which said first separator is in fluid communicationwith said effluent line while said second separator is not, and (ii) asecond position in which said second separator is in fluid communicationwith said effluent line while said first separator is not; and a returnline connecting each of said first and second separators to saidreaction vessel so that liquid or supercritical reaction medium isreturned to said reaction vessel from said separators while solidpolymer is retained in said separator; and control means operativelyassociated with said return line for maintaining said reaction medium asa liquid or supercritical fluid in said first and second separators;whereby effluent from said continuous reaction vessel can be (i)continuously passed through said first separator while polymer may beremoved from said second separator by switching said inlet control valveto said first position, and (ii) continuously passed through said secondseparator while polymer may be removed from said first separator byswitching said inlet control valve to said second position
 16. Anapparatus according to claim 15, wherein said first and secondseparators are filters.
 17. An apparatus according to claim 15, whereinsaid first and second separators are cyclone separators.
 18. Anapparatus according to claim 17, further comprising filters respectivelypositioned downstream of and in fluid communication with each of saidcyclone separator.
 19. An apparatus according to claim 15, wherein atleast one of said separators comprises a plurality of filters inparallel with each other.
 20. An apparatus according to claim 15,wherein at least one of said separators comprises a rotating devicetherein.
 21. An apparatus according to claim 15, further comprising acooler positioned on said effluent line between said reaction vessel andsaid control valve.
 22. An apparatus according to claim 15, furthercomprising a recirculation pump positioned on said return line betweensaid first and second separators and said reaction vessel.
 23. Anapparatus according to claim 15, further comprising a condenserpositioned on said return line between said first and second separatorsand said reaction vessel.
 24. An apparatus according to claim 15,further comprising: an outlet control valve connected to said returnline; and an outlet line connecting each of said separators to saidoutlet control valve; said outlet control valve switchable between (i) afirst position in which said first separator is in fluid communicationwith said return line while said second separator is not, and (ii) asecond position in which said second separator is in fluid communicationwith said return line while said first separator is not.
 25. Anapparatus according to claim 24, further comprising: control means forconcurrently switching said inlet and outlet control valves to saidfirst positions; and concurrently switching said inlet and outletcontrol valves to said second positions.
 26. An apparatus according toclaim 15, wherein said reaction vessel is a stirred tank reactor.
 27. Anapparatus according to claim 15, wherein said reaction vessel is anideal stirred tank reactor.
 28. An apparatus according to claim 15,wherein said reaction vessel is a continuous loop reactor.
 29. Anapparatus according to claim 15, wherein said reaction vessel has adimensionless exit age distribution function (E(Θ)) which reaches amaximum value between about Θ=0 and about Θ=0.3 and thereafter declinesmonotonically after reaching its maximum value.
 30. An apparatusaccording to claim 15, wherein said reaction vessel is configured toprovide a cumulative exit age distribution (F) of from about 0.45 toabout 0.70 when Θ=1.